4-Hydroxy-4-(3-indolylmethyl)-2-aminoglutaric acid (sometimes referred to as “monatin” hereinafter) in the form of (2S, 4S) as represented by the formula (5) is contained in the root skin of a plant Schlerochitom ilicifolius naturally occurring in a northern district of South Africa, namely Northern Transvaal. It is known that the compound in the form has a sweetness level several hundreds fold that of sucrose and is an amino acid derivative useful as a sweetener (see the official gazette of JP-A-64-25757).

Monatin has two asymmetric carbon atoms at positions 2 and 4 and therefore includes the presence of four types of optical isomers.

Various reports tell about methods for producing monatin (see for example Tetrahedron Letters, 2001, Vol. 42, No. 39, p 6793-6796; Organic Letters, 2000, Vol. 2, No. 19, p 2967-2970; Specification of U.S. Pat. No. 5,994,559; Synthetic Communication, 1994, Vol. 24, No. 22, p 3197-3211). Some examples tell about examinations of methods for producing optically active monatin. However, it cannot be said that these methods require such a great number of steps for the production that the methods are not methods industrially suitable.
Alternatively, the present applicant has recently found and has reported a method for producing a particular optically active monatin including a step of synthetically preparing a monatin precursor from indole-3-pyruvic acid to form a diastereomer salt with a particular optically active amine and a step of finally producing the particular optically active monatin via optical resolution (see the pamphlet of the International Application WO 03/059865). For example, (2R, 4R) monatin is listed. The method is represented by the following scheme.

The production method requires a smaller number of steps compared with the traditional methods, by which an optically active monatin can be produced in an efficient manner and which is therefore a production method industrially suitable. However, the unintended optical isomer thereof at position 2 is eliminated into the mother liquid during the crystallization step. When the optical isomer can be isomerized and converted into the intended optically active monatin, the efficiency can be further raised.
As racemization methods of optically active amino acids, for example, methods by treatment under conditions with strong acids and strong alkalis or at higher temperature or racemization methods under relatively mild conditions in the presence of aldehydes have been known. According to these methods, the maximum yields are about 50%. Therefore, these methods cannot be said as efficient racemization methods. A method of racemization and deposition has been known for producing an intended optical isomer in combination with a particular optical isomer at a higher yield (see Tetrahedron, 1997, Vol. 53, No. 28, p 9417-9476). Enormous trials and errors are essential for finding out the combination.